Fiber optic transmission systems are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. A common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect where the different wavelengths within a signal channel travel through an optical fiber at different speeds, e.g., shorter wavelengths travel faster than longer wavelengths or vice versa. This problem becomes more acute for high speed data transmission systems that operate at bit rates of 40 Gbps or higher, where the bit slots are narrower in time and the signal channels are wider in wavelength, since a pulse propagating in one bit slot may broaden and overlap with the adjacent bit, thus causing significant bit errors. In this case, it is necessary to compensate the dispersion before the pulse enters a receiver by recompressing the pulse. This correction is commonly achieved in the prior art by using either an optical filter (e.g., thin-film Fabry-Perot or Bragg grating) or including a section of specially-designed dispersion compensating fiber in the transmission system.
An additional problem is that the dispersion of an optical link can change over time. One common source of this time-dependence is ambient temperature fluctuations, which (due to the thermo-optic effect) cause the index of refraction of the fiber material to change and the corresponding dispersion relation to uniformly shift. At high bit rates, the time slot available for each bit is obviously much smaller than at lower bit rates, so the tolerance for such time-dependent changes in high bit rate systems is extremely low. Thus, in order to ensure that the dispersion is neither under- or over-compensated, it has become necessary to utilize a tunable dispersion compensator in high bit rate systems.
One exemplary tunable dispersion compensator is disclosed in U.S. Pat. No. 6,275,629, issued to B. J. Eggleton et al. on Aug. 14, 2001. In this arrangement, an optical waveguide grating with adjustable chirp is maintained in thermal contact with an electrically controllable heat-transducing body that varies the temperature along the length of the grating. The heat-transducing body can generate heat or remove heat from the grating to form a linear temperature gradient along the grating. By varying the voltage applied to the heat-transducing body, the refractive index of the fiber material is changed, thus changing (tuning) the dispersion compensation.
The Eggleton et al. arrangement is typically limited to a single channel arrangement and is therefore not compatible with reconfigurable optical networks that utilize “colorless” (i.e., operable at various signal wavelengths) receivers.